Coherent illumination is used in electrophotographic printing for image formation on photoreceptors. Unfortunately, the use of coherent illumination sources in conjunction with multilayered photoreceptors results in a print quality defect known as the "plywood effect" or the "interference fringe effect." This defect consists of a series of dark and light interference patterns that occur when the coherent light is reflected from the interfaces that pervade multilayered photoreceptors. In organic photoreceptors, primarily the reflection from the air/charge transport layer interface (i.e., top surface) and the reflection from the undercoat layer or charge blocking layer/substrate interface (i.e., substrate surface) account for the interference fringe effect. The effect can be eliminated if the strong charge transport layer surface reflection or the strong substrate surface reflection is eliminated or suppressed.
Methods have been proposed to suppress the air/charge transport layer interface specular reflection, including roughening of the charge transport layer surface by introducing micrometer size SiO.sub.2 dispersion and other particles into the charge transport layer, applying an appropriate overcoating layer and the like.
Methods have also been proposed to suppress the intensity of substrate surface specular reflection, e.g., coating specific materials such as anti-reflection materials and light scattering materials on the substrate surface and roughening methods such as dry blasting and liquid honing of the substrate surface. For example, photoreceptor substrate surfaces have been roughened by propelling ceramic and glass particles against a surface.
Conventional photoreceptors are disclosed in the following patents, a number of which describe the presence of light scattering particles in the charge blocking layer: Yu, U.S. Pat. No. 5,660,961; Yu, U.S. Pat. No. 5,215,839; and Katayama et al., U.S. Pat. No. 5,958,638.
A problem with conventional charge blocking layers employing light scattering particles is that the range of suitable materials for the light scattering particles is somewhat limited. Many polymeric materials have the particle size, density, and dispersion stability in the proper range, but they have refractive index values that are too close to the binder resin used in the charge blocking layer. Light scattering particles having a refractive index similar to the binder refractive index may produce light scattering insufficient to eliminate the plywood effect in the resulting prints. Selecting inorganic particles such as metal oxides, which typically may have a higher refractive index than polymeric materials, to be the light scattering particles is problematic. This is because inorganic particles such as the metal oxides generally may have higher densities than polymeric materials which can create a particle settling problem that adversely affects the uniformity of the blocking layer and the quality of the resulting prints. Thus, there is a need for an improved charge blocking which avoids or minimizes the problems discussed above.
The phrases "charge blocking layer" and "blocking layer" are generally used interchangeably with the phrase "undercoat layer."